This invention relates to an arrangement of optical isolator components and to a method of tuning an optical isolator to have a maximum isolation for an input beam of light having a predetermined wavelength.
Optical isolators are one of the most ubiquitous of all passive optical components found in most optical communication systems. Optical isolators are generally used to allow signals to propagate in a forward direction but not in a backward direction. These isolators are often used prevent unwanted back reflections from being transmitted back to a signal""s source. It is generally known that optical isolators are to some extent, wavelength dependent devices. They provide a greater amount of isolation for some wavelengths of light and less isolation for other input wavelengths of light.
One prior art polarization independent optical isolator is described in U.S. Pat. No. 5,033,830 issued Jul. 23, 1991 in the name of Jameson and entitled Polarization Independent Optical Isolator. Jameson describes an isolator having a single birefringent plate, a pair of stacked reciprocal rotators, a Faraday rotator, and a reflector positioned in tandem adjacent to the birefringent plate. In a forward (transmitting) direction, a lightwave signal exiting an optical fiber is split into a pair of orthogonal rays by the birefringent plate. The orthogonal rays then pass through a first reciprocal rotator and the Faraday rotator which provides 22.5xc2x0 of rotation. The rotated rays are then redirected by the reflector back though the Faraday rotator. After passing through the second reciprocal rotator, the orthogonal rays re-enter the same birefringent plate where they are recombined and launched in an output fiber. Since a Faraday rotator is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through the birefringent plate such that neither will intercept the input fiber.
An isolated optical coupler is disclosed in U.S. Pat. No. 5,082,343 in the name of Coult et al. issued Jan. 21, 1992. The coupler described in the patent is comprised of a pair of lenses having a wavelength selective device and an isolator disposed therebetween.
Another optical isolator which attempts to improve upon Coult""s design is described in U.S. Pat. No. 5,594,821, issued Jan. 14, 1997 in the name of the applicant, Yihao Cheng.
Yet another optical isolator is described in U.S. Pat. No. 5,267,078 in the name of Shiraishi et al.
Unfortunately, as mentioned heretofore, optical isolators are wavelength dependent devices; as the wavelength of input light varies, the ability of an isolator to provide isolation varies. Thus providing uniformity when manufacturing a large number of devices remains challenging and is often costly. Typically, in the manufacture of isolators, the thickness of components used, such as Faraday rotators, and waveplates, is precisely selected so as to provide maximum isolation for a particular wavelength. Thus, one drawback is that components must be within particular tolerances to provide adequate isolation within predetermined limits at a given wavelength; on the other hand, same components having a different dimensions are required in the manufacture of optical isolators that will operate efficiently at a plurality of different wavelengths. Using prior art methods of construction and manufacture of these devices, once the components are cut to specific dimensions, the isolation response for a given input wavelength of light can be calculated and hitherto, has not been variable within substantial limits.
It is an object of this invention to overcome some of the aforementioned limitations.
It is a further object of the invention to provide a method for fine tuning the maximum isolation response of an isolator with respect to wavelength so the response can be varied within predetermined limits.
Another aspect of the present invention relates to a method of tuning a polarization independent optical isolator having an isolating portion comprising a first polarizing element, a second polarizing element and a polarization-rotating element fixedly disposed there between for passing a beam of light there through. The isolating portion has a maximum isolation response at a predetermined wavelength of light launched into the optical isolator along a first axis. The method comprises the steps of:
varying the angle at which the beam of light is incident upon the isolating portion by rotating the isolating portion about an axis other than the first axis:
launching a beam of light into an end of the optical isolator directed at another end so that the beam of light traverses the isolating portion; and
rotating all of the elements in the isolating portion about the longitudinal axis thereof, simultaneously, to tune the optical isolator until a desired wavelength is measured for a substantially maximum isolation peak.
Another aspect of the present invention relates to a polarization independent isolator comprising: an input port for transmitting a beam of light; an isolating portion; and an output port for receiving the beam of light after traversing the isolating portion. The isolating portion comprises a first polarization element, a second polarization element, and a polarization-rotating element, all or which form a single unit adapted to provide a substantially maximum isolation response in a reverse direction from an output end towards an input end for a given wavelength of light, launched therethrough substantially along a first axis thereof. The isolating portion is oriented so that the beam of light passes through the isolating portion along a line other than the first axis, whereby the polarization independent isolator provides a substantially maximum isolation response at a wavelength other than the given wavelength.
Another feature of the invention relates to a method of tuning a polarization independent optical isolator, having an input end, an output end and an isolating portion. The optical isolator is designed to provide a maximum isolation response in a reverse direction from the output end towards the input end when light at a first wavelength is launched into the optical isolator. The isolating portion comprises a first polarizing element, a second polarizing element and a polarization rotating element disposed there between. The method comprises the steps of: p1 a) launching a beam of light at a second wavelength into an end of the optical isolator and directed at another end so that the beam of light at the second wavelength traverses the isolating portion; and p1 b) tilting the isolating portion with respect to said beam of light to tune the optical isolator until a substantially maximum isolation response is obtained at the second wavelength.